1. Field of the Invention
The present invention relates to thermal and thermal transfer label printers and, more specifically, to such printers adapted to printing on pressure sensitive adhesive-backed labels.
2. Description of the Prior Art
Thermal and thermal transfer printers are well known in the art. In thermal label printers, a web of pressure sensitive adhesive-backed labels, each having a thermally sensitized surface, is fed between a platen roller and a thermal print head. In a thermal transfer label printer, a transfer ribbon having a heat transferrable ink layer is additionally interposed between the print head and the label so that non-sensitized labels may be printed. The transfer ribbon is flexible and typically no thicker than ten microns. Thus, the principles of the present invention are equally applicable to thermal and thermal transfer printers.
Pressure sensitive adhesive-backed labels for automated printing are typically presented in a continuous web. The web consists of a backing sheet of wax or silicone-impregnated paper approximately 0.0015" thick and having multiple labels of paper, polyester, synthetic paper, or similar material having a thickness between 0.0015" and 0.010" removably mounted thereon with a rubber or acrylic pressure-sensitive adhesive. Successive labels are separated by an interlabel gap, typically 0.125" wide, to which the printer is responsive for alignment of printing on the label. The web may be supplied from a roll or from a fanfold.
It is preferable to friction feed the web by driving the platen roller so as to avoid tractor holes in the web which result in increased waste. In a friction fed thermal printer, deformation of the platen roller and slippage between the backing material and the platen introduce variability in the feed distance of the web per increment of platen shaft motion. Slippage is a function of the web tension and produces a net loss in web advance when the printer advances a label against supply roll inertia to facilitate removal after printing and then backfeeds into a slack web before printing the next label.
The error in web advance accumulates as successive labels are printed, resulting in progressive misregistration of the label image with respect to the label edges. A friction fed printer thus requires some means of sensing the edge of each label for synchronization in order to print multiple labels without manual intervention.
Label location in typical prior art thermal printers has been accomplished by measuring the optical transmissivity of the web. The backing is illuminated by a light source of known intensity, typically an infrared light-emitting diode. The amount of light passing through the backing between labels is greater than the light passing through the laminated backing and label. The transmitted light illuminates a photocell, which converts the changes in transmitted light to a varying electrical signal. The electrical signal can then be measured and interpreted as the label edge location by the printer's logic circuits and used to synchronize printing of each label.
The optical sensor just described has inherent limitations. Even though the intensity of the light source is constant, the paper fibers in the label and in the backing produce fluctuations in the light intensity which introduce error in the edge determination. Additionally, transverse movement of the slack web perpendicular to its plane between the light source and the photocell occurs during backfeed, which introduces an additional error.
The optical sensor is typically located an inch or more away from the heater elements to avoid mechanical interference with the print head or platen. If the web slips between the time the leading edge of a label passes the photocell and when it reaches the heater elements, or if slack develops between the photocell and the heater elements during backfeed, the printing will be misregistered on the label.